ADENOSINE 5'-[ALPHA-BETA-METHYLENE]TRIPHOSPHATE POTENTIATES THE OSCILLATORY CYTOSOLIC CA2+ RESPONSES OF HEPATOCYTES TO ATP, BUT NOT TO ADP

Single rat hepatocytes microinjected with aequorin generate oscillations in cytosolic free Ca2+ concentration ([Ca2+]i) when stimulated with agonists acting through the phosphoinositide signalling pathway. The duration of these transients has been shown to be characteristic of the stimulating agonist, so that transients of very different duration can be induced in the same individual hepatocyte by different agonists. In a previous study we have shown that ADP and ATP, which are believed to act through a single P2y-purinoceptor species, elicit very different [Ca2+], responses in most of the hepatocytes. We have interpreted this as evidence for two Ca2+-mobilizing purinoceptors. The methylated derivative of ATP, adenosine 5'-[alphabeta-methylene]-triphosphate (pp[CH2]pA), is only a weak P2y-purinoceptor agonist. When 100 muM pp[CH2]pA was supplied to aequorin-injected hepatocytes, there was no effect on [Ca2+]. However, 25 muM pp[CH2]pA co-supplied with ATP causes a potentiation of the [Ca2+]i response in most of the hepatocytes. The effect was specific for ATP-induced transients; [Ca2+]i transients induced by other agonists, and importantly by ADP, were not affected by addition of pp[CH2]pA. This further illustrates differences in the actions of ADP and ATP, strengthening the argument for separate receptors for these nucleotides.